Hard disk drives store information on one or more disks or platters that rotate about a spindle with respect to one or more heads, such as read and/or write heads. Write heads are highly inductive devices that are controlled by write circuitry that controls current through the write head to create local electromagnetic fields that magnetize portions of a disk drive platter to impart information thereto. For example, the hard drive write circuitry may pass current through the write head in a first direction to write a first logical data bit (e.g., a logical one) to a platter. Conversely, the hard drive write circuitry may cause current to pass through the write head in a second direction opposite the first direction to write a second logical data bit (e.g., a logical zero) to the platter. Because the current through the write head must change direction to write different bits of information to the platter and because fast data write speeds are needed, it follows that, when writing information to a platter, there is a significant change in current through the write head in a small period of time, which results in a large change in current per unit of time (i.e., a large dI/dt).
Hard drive write circuitry, which may be implemented in an open loop H-bridge topology, is coupled to a write head through a transmission line. The highly inductive nature of a hard drive write head means that hard drive write circuitry may consume significant amounts of power given the fast data speeds at which information is written to platters in hard drives. That is, the hard drive write circuitry is required to change current though the write head rapidly to accommodate fast hard drive write circuitry.
Rapidly changing the current through an inductive hard drive write head requires impedance matching within the hard drive write circuitry because of the large current swings in the write head. At high data rates, the impedance matching is conventionally implemented as an open control loop. Such impedance matching circuits consume significant amounts of power to maintain the impedance match between the hard drive write circuitry and the write head while absorbing initial power reflection resulting from the current launched into the inductive head that is required to energize the write head. At high data rates, having large changes in current per unit of time (dI/dt) are required to drive the write head. In such cases, the open loop impedance matching circuitry saturates and, thus, the strength of the impedance match provided by the same weakens. The weakening of the impedance match between the write circuitry and the write head causes the write circuitry to lose control of the peak current launched into the write head. Impedance matching using known closed control loops is insufficiently fast to keep up with the operating speed of an open loop H-bridge write drive topology.